Power/ground metallization routing in a semiconductor device

ABSTRACT

A semiconductor device and a method of laying out the same includes routing primary power and ground distributions in the second metallization layer, rather than the first metallization as is conventionally done. This improves routability in the first metallization layer while providing sufficient current handling ability in the power and ground distributions.

This is a continuation of application Ser. No. 08/984,029 filed Dec. 2,1997 now U.S. Pat. No. 5,981,987.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor integrated circuits, andmore particularly, to power and ground metallization routing in amulti-metal layer semiconductor device having a plurality of basic cellcircuits such as standard cells and gate array cells.

2. Description of the Related Art

FIG. 1 illustrates a conventional integrated circuit having a number ofrows 3 of cells 5. The cells can have various widths W1, W2, W3, etc.and can be separated by small gaps (not shown). Power and ground aresupplied to each cell from power and ground busses 7 and 9 via primarypower and ground distributions 60 and 50, respectively. The primarypower and ground distributions are typically laid out in the firstmetallization layer (i.e., “metal 1”). Moreover, metals in adjacentlayers are laid out perpendicular to each other. That is, for example ina four-metal layer integrated circuit, wirings in the first and thirdmetallization layers are laid out in one direction, and wirings on thesubstrate surface (e.g. polygate) and the second and fourthmetallization layers are laid out in a direction perpendicular to thewirings in the first and third metallization layers.

As integration increases, rows 3 begin to abut with each other, causingthe distance D1 to shrink to such a degree that the availability of thespace between rows as channels for routing interconnections betweencells in metal 1 is eliminated. Over-the-cell routers and other toolsare thus required to route such interconnections in higher metal layers.

FIG. 2 illustrates the layout of a basic cell 5 that can be included insuch a conventional integrated circuit as is illustrated in FIG. 1. Itincludes a PFET device region 10, a NFET device region 20, polygate 30,P-N device intraconnection 40, primary ground distribution 50, andprimary power distribution 60. Contacts 70 connect power from primarypower distribution 60 to the PFET device region, and contacts 80 connectground from primary ground distribution 50 to the NEFT device region.Input pints 85 are provided to connect devices in this cell with devicesin other cells by contact to polygate 30 through contact 95.

As can be seen, the primary power and ground distributions are laid outin metal 1 in an east-west direction. P-N intraconnections 40 and inputpints 85 are also typically laid out in the first metallization layer.As should be apparent, to connect devices in cells in other rows to theinput pins 85 and output pins (typically via connection to P-Nintraconnection 40) of cell 5, such connections must be routed up andover the primary power and ground distributions through higher metallayers and then back down to metal 1 through vias and contact holes andthe like.

FIG. 3 is a side plan view of the basic cell in FIG. 2 taken alongsectional line 3—3. It shows primary power distribution 60 formed as thefirst metal layer over PFET device region 10, with polygate 30 (i.e., agate formed of a layer of doped polysilicon on the substrate) and firstinsulator layer 90 interposed therebetween. Device region 10 is formedin substrate 1 and is separated from other device regions by oxide 35.Gate oxide layer 25 is interposed between polygate 30 and device region10. Input pin 85 is connected to polygate 30 by contact 95 through firstinsulator layer 90.

The conventional technique of routing primary power and grounddistributions in metal 1 is fraught with many problems. First, forexample, due to the requirement of providing P-N intraconnections suchas 40, and the fact that cell integration restricts the availability ofcell interconnections between rows, very few cell interconnections canbe routed in metal 1. Meanwhile, it is generally desirable to route asmany interconnections as possible in lower metal layers so as toconserve routing resources in upper metal layers, and thus facilitatereduced average wire lengths.

Second, as cell integration increases, the number of devices per squarearea of the die increases, and hence the amount of current required tobe carried on the primary power and ground distributions increasesbeyond the capabilities of the distribution lines. One solution to thisproblem involves making the primary power and ground distributionswider. However, certain minimum design distances such as D2 and D3 mustbe maintained so as to comply with the minimum feature requirements ofthe fabrication tools, for example. If the power and grounddistributions are made wider, the device regions themselves mustlikewise be made wider, thus defeating higher cell integration.Moreover, an imbalance problem can arise even if the minimum featurerequirements are maintained by increasing the size of a N device region,but without increasing the size of a P device region by a correspondingamount. This is because P devices are typically such weaker than Ndevices.

A second solution to the above-described current handling probleminvolves adding supplemental lines in metals 2 or 3.

FIG. 4 illustrates the technique of laying out supplemental line 110 inan east-west direction in metal 3 in parallel with primary powerdistribution 60 in metal 1. The primary and supplemental lines areconnected through second insulator 100 and third insulator layer 105 byperiodically provided stacked via and contacts 120. This solutioneffectively increases the width of the primary power distribution line.However, this effective increase in width may not be sufficient inextreme circumstances where many cells in the same row require currentat the same time. Moreover, cells may have different dimensions, causingthe primary distribution line to snake north and south and making itdifficult to align the primary and supplemental lines.

FIG. 5 illustrates the technique of providing supplemental lines inmetal 2. In this technique, supplemental power lines 115 are laid out inmetal 2 in a north-south direction forming a matrix with the underlyingprimary power distributions. Inter-layer contacts are periodicallyprovided to connect the supplemental power lines 115 and primary powerdistribution lines 60. This technique permits the current in each of theprimary power distributions 60 to be shared in parallel so that a “hot”row of devices can draw current from other primary power distributions60 associated with other rows. It should be apparent from the foregoingthat the same technique could be applied for ground as well as power.

Although providing supplemental lines in metal 2 improves the ability ofthe primary power and ground distributions to provided desired amountsof current, other problems are created. For example, the supplementalline 115 in metal 2 can interfere with metal 1 pin locations and thuscan prevent picking up device input and output pins. This furtherproblem is illustrated in FIG. 6. As can be seen, when supplemental line115 is laid out as shown in dashed lines, pin 85 is blocked, preventingany connection thereto unless a metal 1 interconnection can be made,which is unlikely. Accordingly, either the cell must be made wider orgaps must be provided between cells over which to lay out thesupplemental line 115, as shown in FIG. 6. FIG. 7 is a side view of thecell in FIG. 6 taken along sectional line 7—7. As should be clear,either widening the cell or providing larger gaps between cells defeatshigher cell integration.

Accordingly, there remains a need in the art for effective primary powerand ground distributions in a basic cell that provides sufficientcurrent handling ability while not impeding metal 1 routability orincreased integration. The present invention fulfills this need.

SUMMARY OF THE INVENTION

An object of the present invention is to provide effective primary powerand ground distributions in an integrated circuit having a plurality ofcells.

Another object of the invention is to provide sufficient currenthandling ability of power and ground distributions in an integratedcircuit having a plurality of cells.

Another object of the invention is to improve device interconnectionroutability in an integrated circuit having a plurality of cells.

Another object of the invention is to improve cell integration.

Another object of the invention is to improve the ability to providesupplemental lines to primary power and ground distributions.

Another object of the invention is to improve P/N device balancing.

Another object of the invention is to reduce average wire lengths.

These and other objects of the invention are fulfilled by the presentinvention. In a preferred form, the invention includes primary power andground distributions in the second metallization layer, rather than thefirst metallization as is conventionally done. This improves routabilityin the first metallization layer while providing sufficient currenthandling ability in the power and ground distributions.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages the present invention, among others,will become apparent to those skilled in the art after considering thefollowing detailed specification, together with the accompanyingdrawings wherein:

FIG. 1 illustrates the layout of a conventional integrated circuithaving rows of cells;

FIG. 2 illustrates the layout of a basic cell in a conventionalintegrated circuit such as that illustrated in FIG. 1;

FIG. 3 is a side view of the conventional cell in FIG. 1 taken alongsectional line 2—2;

FIG. 4 illustrates the conventional technique of providing supplementalpower and ground distributions in metal 3 in the conventional integratedcircuit;

FIG. 5 illustrates the conventional technique of providing supplementalpower and ground distributions in metal 2 in the conventional integratedcircuit;

FIG. 6 further illustrates the conventional technique of providingsupplemental power and ground distributions in metal 2 in theconventional integrated circuit;

FIG. 7 is a side view of the conventional cell in FIG. 6 taken alongsectional line 7—7;

FIG. 8 illustrates the layout of a basic cell with power and grounddistribution routing in accordance with the present invention;

FIG. 9 is a side view of the basic cell illustrated in FIG. 8 takenalong sectional line 9—9;

FIG. 10 illustrates providing supplemental lines in metal 3 and metal 4in accordance with the principles of the present invention;

FIG. 11 illustrates inter-cell connections in metal 1 in accordance withthe principles of the present invention;

FIG. 12 illustrates providing substrate and well ties in an integratedcircuit in accordance with the invention.

FIG. 13 illustrates a multi-height basic cell in accordance with theprinciples of the present invention; and

FIG. 14 further illustrates providing multi-height basic cells in anintegrated circuit in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 8 illustrates the layout of a basic cell layout using power andground distribution routing in accordance with the present invention. Itincludes PFET device region 10, NFET device region 20, polygate 30, P-Ndevice intraconnection 240, primary ground distribution 250, primarypower distribution 260, cell output interconnection 242 and cell inputinterconnection 244. Stacked via and contact hole 270 connects powerfrom primary power distribution 260 to the PFET device region, as willbe described in more detail below.

Contrary to the conventional techniques, the primary power and grounddistributions are formed as the second metallization layer in the basiccell, and are routed in an east-west direction. P-N deviceintraconnection 240, cell output interconnection 242, and cell inputinterconnection 244 are formed in the first metallization layer, and canbe routed in both north-south and east-west directions. Other elementscan be the same as in the conventional cell and their repeated detailedexplanation here is not necessary for an understanding of the invention.

As should be apparent, the routability of inter-cell connections inmetal 1 is enhanced due to the lack of primary power and grounddistributions in metal 1. In the example illustrated in FIG. 8, P-Ndevice intraconnection 240 can be connected to cell outputinterconnection 242 so as to provide the output of this cell to a cellin a northern row in metal 1, while input pin can be connected to cellinput interconnection 244 so as to supply the input of this cell from acell in a southern row. Other examples and alternatives of connectingthe inputs and outputs of cells in metal should be immediately apparentto those skilled in the art.

As should be further apparent, cell integration can be dramaticallyimproved using the power and ground distribution routing in accordancewith the invention. Not only does the improved routability of cellinterconnections in metal 1 conserve routing resources in higher metallayers and reduce average wire lengths, but the device regions can bemade smaller due to the ability, for example, to overlap portions of P-Ndevice intraconnection 240 with primary power and ground distributions250 and 260. Further, the N device region can be made smaller relativeto the P device region, thus allowing for better P/N balance.

FIG. 9 is a side plan view of the basic cell in FIG. 8 taken alongsectional line 8—8. It shows primary power distribution 260 formed asthe second metallization layer over PFET device region 10, with firstinsulator layer 90 and second insulator layer 100 interposedtherebetween. It also further graphically illustrates how cell outputinterconnection 242 can be freely routed in metal 1 to connect devicesin the basic cell in FIG. 8 via contact 210 with other cells north andsouth of the cell.

The primary power and ground distributions can be connected to therespective device regions using many known techniques. However, in apreferred embodiment of the invention illustrated in FIG. 8, primarypower distribution 260 is connected to the PFET device region throughstacked via and contact 270. By using a stacked via and contact such asthat illustrated, the use of metal 1 is minimized, thus furtherimproving the routability of other interconnections in metal 1.

Further advantages of routing the primary power and ground distributionsin metal 2 rather than in metal 1 are as follows. First, the power andground distributions in metal 2 can be made as wide as necessary tohandle the current required to supply the integrated circuit devices.Moreover, metal 2 layers are trending toward being thicker than metal 1,further enhancing the current capacity of the power and grounddistributions in metal 2.

As illustrated in FIG. 10, if supplemental power and grounddistributions are still required, supplemental lines 215 can be providedin metal 3 in a matrix fashion with the primary distributions in metal2, with periodic connections therebetween. Furthermore, secondsupplemental lines 217 in metal 4 can be further provided in a matrixfashion with the supplemental lines in metal 3, with periodicconnections therebetween. It should be apparent that the pin blockingproblem described with reference to FIG. 6 is alleviated in the presentinvention by the ability to access pins in metals 1 and 2.

FIG. 11 illustrates how cells 5-A and 5-B in neighboring rows can beinterconnected with each other and with other cells in metal 1 inaccordance with the principles of the invention. This example shows theoutput of cell 5-A connected with the input of cell 5-B by cellinterconnection 342, while other inputs of both cells are connected withcells in the same and other rows by cell interconnections 344, 346 and348.

Although substrate and well ties can be provided in many known ways,FIG. 12 illustrates providing substrate and well ties in a mannerpreferred by the present invention. In the example shown in FIG. 12,substrate ties 303 and well ties 304 are provided at the corners ofevery cell, with adjacent cells in the same row sharing the samesubstrate and well ties, so as to connect ground and power respectivelyto substrate and N-wells in each cell. By providing the substrate andwell ties in this manner, routability in metal 1 in both north-south andeast-west directions is not significantly impeded.

Yet another advantage of the primary power and ground distributionrouting of the present invention is illustrated in FIG. 13. By virtue ofthe improved routability of cell interconnections in metal 1,multi-height cells can be more easily provided that before. FIG. 13shows an example of a double-height cell 305 linked together by deviceintraconnection 440 in metal 1. Double-height cell 305 can be considereda stronger version of the basic cell illustrated in FIG. 8, with moreinput and output pin locations, thus further enhancing the routabilityof interconnections in metal 1. FIG. 14 further illustrates how amulti-height cell such as double-height cell 305 can be provided in anintegrated circuit having a plurality of single-height cells 5. Thisadvantage of the invention is particularly important for integratedcircuit designs where complicated cell structures having many input andoutput pins are required. FIGS. 13 and 14 also illustrate anotherexample of how substrate and well ties 303 and 304 are provided inaccordance with the invention.

It should be noted that although the routing techniques of the presentinvention have been described hereinabove with particular reference tointegrated circuits having standard cells, the principles of theinvention can also be applied to gate arrays having predetermined basicgate array cells.

Accordingly, although this invention has been described in detail withreference to the preferred embodiments thereof, those skilled in the artwill appreciate that various substitutions and modifications can be madeto these examples without departing from the spirit of the invention asdefined in the appended claims.

We claim:
 1. A basic cell in an integrated circuit having a plurality ofcells, said basic cell including: a first conductivity device region; asecond conductivity device region; a first metal layer formed over saidfirst and second conductivity device regions, said first metal layercomprising interconnections between devices formed in said first andsecond conductivity device regions and other of said plurality of cells;a second metal layer formed over said first metal layer, said secondmetal layer comprising primary power and ground distributions; and meansfor conducting current respectively between said primary power andground distributions and said first and second conductivity deviceregions.
 2. A basic cell as defined in claim 1, wherein said meansconducting current comprises a stacked via and contact.
 3. A basic cellas defined in claim 1, wherein said primary power and grounddistributions are routed in a first direction, and said interconnectionsare routed in said first direction and in a second direction differentthan said first direction.
 4. A basic cell as defined in claim 1,wherein said basic cell is a standard cell.
 5. A basic cell as definedin claim 1, wherein said basic cell is a gate array cell.
 6. A basiccell as defined in claim 1, further comprising: a third metal layerformed above said second metal layer, said third metal layer comprisingsupplemental power and ground lines, said primary power and grounddistributions being routed in a first direction and said supplementalpower and ground lines being routed in a second direction different thansaid first direction; and means for conducting current respectivelybetween said supplemental power and ground lines and said primary powerand ground distributions.
 7. A basic cell as defined in claim 6, furthercomprising: a fourth metal layer formed above said third metal layer,said fourth metal layer comprising second supplemental power and groundlines, said second supplemental power and ground lines being routed insaid first direction; and means for conducting current respectivelybetween said second supplemental power and ground lines and saidsupplemental power and ground distributions.
 8. A layout method in anintegrated circuit having a plurality of cells, each of said cellsincluding a first conductivity device region and a second conductivitydevice region, said method comprising: providing interconnectionsbetween said cells in a first metal layer formed over said first andsecond conductivity device regions; providing primary power and grounddistributions in a second metal layer formed over said first metallayer; and providing means for conducting current respectively betweensaid primary power and ground distributions and said first and secondconductivity device regions.
 9. A method as defined in claim 8, whereinsaid means providing step comprises providing a stacked via and contactbetween said power and ground distributions and said first and secondconductivity device regions.
 10. A method as defined in claim 8, furthercomprising: routing said primary power and ground distributions in afirst direction; and routing said interconnections in said firstdirection and in a second direction different than said first direction.